Forget about whether the chicken crossed the road or not. The question for some scientists is why other birds won’t even come near a roadway.

Wherever wildlife and automobile traffic meet, it’s usually bad news for the animals. More than just making roadkill, roads use up land, create noise and pollution, and act as barriers that cut animals off from resources, mates, and territory. Because of all these problems, bird populations tend to decline sharply within a kilometer of a road or other human infrastructure, and mammals begin declining within five kilometers.

You’re within about a kilometer away from a road in more than 80 percent of the USA. That’s a whole lot of country covered in things that bug birds. But what bothers them most? The trash? Close calls with windshields? A lot of research has suggested that it's the level of noise around a busy roadway, but most of these studies have been done along actual roads. When all the potential bird-deterring effects are there at the same time, it’s hard to pin down the strength of any single one.

To isolate the noise factor and see how much it matters to birds, biologists from Boise State University in Idaho wanted a way to create traffic noise without actual traffic. They decided to hide speakers in the trees of a southern Idaho forest that migrating birds use as a rest stop. When they piped recordings of traffic sounds through the speakers, they had a half-kilometer-long “phantom road” that wouldn’t bother the birds any way but through their ears.

The Road Not Taken

Alternating between four days of noise and four days of quiet throughout the fall migration season, the researchers recorded visits of more than 8000 individual birds from 59 species to their phantom road and a noise-less control site. Whenever they turned the speakers on, the total bird abundance at the phantom road declined by more than a quarter. Some species avoided the area in even greater proportions, and a few, like the cedar waxwing, avoided it almost entirely.

Scaring away that many birds with only traffic noise is a startling demonstration of how man-made noise can alter the way animals use space. Because of the sheer amount of land that roads cover in the U.S., particularly noise-sensitive species like the waxwings and yellow warblers are pushed away from a whole lot of otherwise useable habitat because it’s too loud for them.

Even within national parks and other protected areas, the researchers say, roads can produce noise levels similar to their phantom road, and man-made noise needs to be taken into account when preserving and managing land and wildlife. The next step is figuring out why noise is such a big deterrent for birds. It could be that noise masks birds’ songs and calls and keeps them from finding or communicating with one another. Other scientists have found that birds with high-frequency songs aren’t as bothered by roads and certain industrial sites because these low-frequency noises don’t drown out their songs as much as they do some other species.

Road noise might also turn birds away because it keeps them from hearing predators. Some birds, like chaffinches, and other animals are more vigilant in noisy areas so other animals don’t get the drop on them, often at the expense of eating or other normal behaviors. If more noise means less eating, then roads are especially lousy places to be when a bird is migrating and needs fuel to keep going.